You're still missing the point. If the exchange is providing a fiat on/offramp, then it will have to manage the possibility that the amount of USD deposits is massively exceeded by the amount of Bitcoin deposits which may one day call on the USD deposits to be redeemed.

For example. If I bought $1000 for 1 BTC from Coinbase on January 1st 2017 and try to exit that position on Jan 1 2018, once I sell that $10,000 bitcoin to Coinbase I'll be withdrawing $9000 more than I put in. Coinbase has to get the $9000 from somewhere, whether that be from other deposits which are more recent than mine, or from its own trading operations.

If you have a large number of people trying to withdraw a large number of dollars at the same time, your offramp could run out of dollars or, in the alternative, be required to (a) draw down a facility with a bank or (b) sell assets. In an environment where the asset price is falling rapidly it may not be able to avail itself of either option.

As occurred in 2008, with dire consequences.